Process for pickling metal



April 10, 1956 c. A. RAUH PROCESS FOR PICKLING METAL Filed Jan. 7, 1953 IN V EN T 0R.

COR/V6003 .4. R40 Qlkez ewszz United States Patent '0 PROCESS FOR PICHJING METAL Cornelius A. Raul], Akron, Application January '7, 1953, Serial N 3343,62! Claims. (Cl. 134-13) The present invention relates to improvements in the process used for pickling metal and more particularly to a process for the pickling of steel wherein the pickling medium is continually reconditioned.

In the working of steel, for example by rolling operations whereby steel is reduced to strip, sheet or plate form, scale or rust usually are formed on the metal surface. Removal of such rust or scale is necessary to give a desired finished brightness, and at times to condition the steel for further processing steps. The scale or rust has long been removed in the generally used pickling process by subjecting the steel to the action of a sul-v phuric acid solution which reacts with the iron compounds constituting the rust or scale with the attendant formation of iron sulphates. in the pickling process, batches of steel in sheet, plate, or other forms are immersed in tanks of pickling solution, or in the case of continuous strip rolling, the continuous strip is passed through a vat of the pickling solution after the finish roll. Subsequent to the pickling operation the acid solution adhering on the steel must, of course, be removed by suitable rinsing.

To attain the desired pickling activity the pickling solution is commonly used at an elevated temperature, and further, agitation of either thesteel or the pickling solution in the pickling tanks is maintained. The. agitating devices at the tanks constitute encumbrances which often hinder the loading and removal of steel therefrom. Heat loss to the steel passed through the pickling solution or to the surroundings require or render desirable the continual heating of the solution to maintain a suitable temperature. This heating is usually and most simply effected at the tanks by injecting steam into the solution, a method tending to produce a fog or mist of acid solution above the surface of the pickling bath,

which is objectionable on escape to the free atmosphere.

because of its corrosive or acid nature. This method of heating has the further disadvantage that condensation of the large amount of steam used in the normal course of the pickling process greatly increases the volume of the solution in the pickling tanks, thereby necessitating addition of acid to overcome dilution, and requiring oversize tanks or other means to prevent overflow. Of course, as the pickling process itself consumes acid due to the formation of iron sulphate salts, acid must be added to maintain an acid concentration which will give the desired,

pickling activity. During the life of the pickling solution the concentration of dissolved iron salts builds up, thereby tending to diminish the pickling rate in. accordance with the ordinary laws of chemical equilibrium, so that it is necessary to increase the acid concentration to main tain a desired pickling rate. Ultimately the concentration of the iron salts becomes so high that further increase in the acid concentration is not practically effective in overcoming the iron concentration and then the solution is dumped from the tanks and .replaced by fresh solution. Such pickling liquor at the timeof dumping. may, for

2 example, contain by weight 5% sulphuric acid and 15% ferrous sulfate.

Clearly, this dumping procedure involves several disadvantages. The dumping of hot pickling solution and repl'acementby fresh solution subjects the acid resistant tank linings tothe deleterious eflects of thermal shock. There is a loss of unused acid, of heat energy in the hot pickling solution, and economic losses due to the cost of dumping and disposal operations. Due to the large volumes of solution. involved it is uneconomic to evaporate the waste pickle liquor and the usual method of disposal has been dumping of the waste liquor into streams, lakes or sewage systems adjacent the plant; or, in some localities, hauling the large volumes of spent solution by barge for dumping in bodies of Water such as the ocean or large inland lakes where the disposal ofthe solution is less objectionable because of the resultant high degree of dilution. Such dumping procedures, however, are objectionable in the eyes of governmental authorities because of the acid content of the solution and'the high biological oxygen demand of the ferrous salts which adversely affect aquatic life. In many localities it is necessary by government regulation to neutralize the acid and remove the iron compounds, which, of course, entails the cost of labor and of an acid neutralizing and iron precipitating agent such as calcium oxide.

The present invention provides for the continuous reconditioning of pickling solution by continuous withdrawal from pickling tanks for removal of iron salts and heating to a desired temperature, with continuous return of a suitably conditioned solution to the pickling tanks. Hence there is no waste solution to be dumped. Since the pickling medium is in continuous fiow through the pickling tanks the need for agitation of the tank contents can be readily obviated. Further, since heat is supplied to the pickling liquor constituents during the reconditioning operations, there is no need for heating of the tanks. Since tank agitating means are not required the design of pickling tanks can be radically changed from present designs, for no space is required for such auxiliary apparatus. Hence, the agitating devices usually encumbering pickling tanks, and the high solution dilution and corrosive fog or mist usually produced by the common tank heating systems, are eliminated.

The method of this invention is carried out by continuously withdrawing pickling solution from the tanks, a portion thereof being evaporated preferably by direct contact heating, for example by submerged combustion, to a temperature and to a concentration causing the formation of crystalline ferrous sulphate preferably the monohydrate. The evaporated solution is put through a crystallizer to efl ect separation of the ferrous sulphate from the residual strong acid solution, which is ultimately returned'to the pickling solution. The major part of the pickling solution withdrawn from the pickling tanks is passed through'a heating system wherein it absorbs latent and sensible heat energy and preferably water vapor from line gas produced by the aforesaid evaporation. Water and acid, continually added to the system in amounts sutficient to make up the solution consumed by formation of ferrous sulfate and the drag-out losses of solution adhering to steel, may also be heated by thermal energy recovered from the evaporator flue gases. Thus the. heat. required to maintain the desired pickling temperature also serves for evaporation in the removal of the ferrous salts. Consequently in this invention, no dumping of pickleliquor is required and the operational and material costs usuallyattendant thereon are eliminated. The monohydrate salt recoverable in the process is-itselfof economicyaluqas 'it now appears that a decided market is in prospect for the monohydrate preferential to the septahydrate. Even apart from this con-' pickle liquor with an attendant replacement by fresh solution, which usually has a high rate of attack not only on scale and rust but also on clean steel, and as make-up acid is continually supplied to the system, sharp discontinuities in the performance of the pickling vats are eliminated. Further, the presence of residual iron salts in the reconditioned pickling liquor in this invention is controlled to a concentration permitting such salts to serve as an inhibitor of the action on clean steel without deleterious effect on the pickling rate. Thus the loss of clean steel and the consumption of acid therein is controlled. Other advantages of the invention will appear'from the drawings and the following description of the invention.

In the drawing, which is a schematic representation, the battery of iron or steel pickling tanks 1, 2, 3 are continually supplied through inlet lines 4 having individual valves 5 from manifold line 6 with fresh, that is reconditioned, pickling acid solution flowing from liquid outlet line '7 of primary heater 8 hereinafter more fully described, At each tank used pickling solution siphoned through a tank outlet 10 is divided to pass in greatest part through line 11 with valve 12 to manifold 13 connected by line 14 to the inlet of pumps 15 for delivery by the pumps through line 16 to the solution inlet 43 of primary heater 8; and to pass through line 17 provided with valve 18 to a manifold line 19 connected by line 20 to a pickling solution storage tank 21, which may be buried in the ground.

As an example of the pickling solution concentrations be hereinafter described, preferably are of suitable acid resistant construction and materials, and are insulated to minimize loss of heat from solution therein.

The other major components of 'the pickling liquor reconditioning section of the system are an evaporator 39, a crystallizer 31, centrifuge 32, strong'acid storage tank 33 and secondary heater 34. The evaporator 30 preferably is of a direct heating type, wherein the solution comes into direct contact with flame providing the,

heat energy, so that the flame releases energy directly to the solution, with the products of combustion and evaporated moisture being carried off together as evaporator flue gas. A submerged combustion evaporator is shown in the drawing and the process will be described primarily with reference thereto,.but'a tower evaporator wherein Y a spray of solution and flame are brought together, or an evaporator with conventional heating of the solution by heat transferred through a wall from the heating mechanism, may be used.

From storage tank 21 pickling solution drawn from the battery of pickling tanks is pumped through line 36 by pump 37 to the evaporator 30, wherein it is heated to about 240 F. by submerged combustion burner 38' supplied with fuel through fuel line 39 and air through the compressed air line 49. Flue gas at about260 F., comprising the products of combustion and evaporated water pass from the evaporator through hot flue gas duct. 41 to the primary heater 8, wherein the gases pass up:

wardly through a section of suitable packing 42 countercurrently to the relatively cooler solution recirculated from the tanks by pumps 15 and introduced to heater 8 through liquid inlet 43 at temperatures which may bein the range, for example, of l30-l90 F. Water vapor,

which may be 60% by volume or. more in the flue gas, arising by evaporation of water from pickling solutionand where the fuel contains combined or free hydrogen, as

which may be maintained, the aqueous solution leaving the primary heater 8 may be by weight 30% sulfuric acid and 10% ferrous sulfate, while the used acid leaving the Preferably the capacity of tank 21 is great enough to store the entire volume of solution in the system. By means of valves 5, 12 and 18 flow of solution relative to each tank in'the battery may be controlled and, if desired, cut otf at an individual tank of the battery for repairs or other purposes. Further, to permit the solution conditioning section of the system to be operated during anytime when it may be advantageous to shut down all or substantially all of the pickling battery, and also to give greater flexibility of operation, there are a valve 22 between manifold lines 6 and line 7 and a valve 23 between manifold line 19 and line 20 for isolating the pickling battery, a line 24 with valve'25 betweenstorage tank 21 and pump inlet line 14 whereby-solution may be delivered from tank 21 to heater 8, and a by-pass 26 with valve 27 from outlet line 7 to line 20 for by passing heated 7 solution from the heater to the storage tank.

The tanks 1, 2 and 3 of the drawing represent generally a one of the products of combustion in evaporator 30, is in great part condensedby the acid solution circulated through heater 8, thereby allowing the recovery of latent 'heat of evaporation, while cooling of'the other products of combustion results in further sensible heat recovery;

Consequently heating of the recirculated solutionrand cooling of the flue gas is obtained. Hence the solution passing back to the tanks through lines 7 from the primary heater 8 is reheated to a temperature suitable for maintaining the desired pickling conditions, for example, at a temperature of to as much as 200 F., and a great part of water evaporated at evaporator 30 is retained in the process. V

The cooled gas stream from gas outletof heater 8 say at 155 -l90 F. is carried by duct 45 including a packed chamber-46 immediately above the gas outlet of heater 8 to a secondary heater 34 similar to heater 8. Strong acid from the'crystallizer31 is, as hereinafter explained, to be combined with recirculated acid for return to the pickling battery. As this strong acid and make-up acid is delivered through a liquid inlet 47 of the packed chamber 46 to pass downwardly through the packing countercurrently to flue gas, a further absorption of water vapor from the cooled flue gas due to the relatively low water vapor pressure of the strongacid, results in a greater recovery of latent heat of evaporation. The strong acid leaving chamber 46 through duct 45 descends into the primary heater 8 to mix with the acid recirculated directly from the pickling battery by pumps 15. Under some con ditions of operation, due to the release of latent heat of vaporization, when strong acid absorbs water vapor from the flue gas, the flue gas stream may be reheated in 46. In secondary heater 34 partially cooled flue gas is further cooled before venting to the atmosphere through outlet 49 by passing upwardly through the packed section 50 thereof countercurrently to water introduced at inlet 51, the resultant heated water, which may be at 135 to F., being conducted from the secondary heater outlet by j line '52 to a hot water storage tank 53. A part of this hot water is fed as make-up water as required through line 54 by a pump (not shown) to line 16 for mixing with the acid circulated to heater 8, and the remainder may be directed through line 55 to hot rinse tanks (not shown) beyond the pickling battery for use in rinsing pickled steel. Make-up acid may be fed continuously at 59a to chamber 46 to replace acid lost as by drag-out at the pickling tanks and the formation of iron sulphates. Although not shown in the drawing, it is to be understood that provision may be made to return used acid-bearing rinse water from the hot rinse tanks to the pickling system at heater 8.

As another form of direct contact heating, a tower type evaporator may be used having a chamber into which a spray of the solution and a flame are injected for direct contact; a packed section wet by a stream of additional solution into which the hot gases produced in the spray chamber are passed to effect concentration of the additional stream and, in some degree, quenching of the hot gases; and a sump for collecting the solution concentrated in the spray chamber and packed section. Aconventional evaporator, such as a Svenson evaporator, may be used but usually with a loss of the advantages of the direct heat ing types. In any case the flue gases from the direct heating evaporators or the water vapor stream produced in the conventional evaporator are directed to the primary heater for use therein as the heating medium.

The primary heater 8 may be of the more conventional type of heat exchanger, wherein the recirculated solution from line 16-with, if desired, strong acid from the lines 59 and 59a-is passed through tubes surrounded with flue gases or water vapor produced in the evaporator to effect heattransfer from the flue gases to the liquid and condensatio'n of water vapor from the flue gases. The resulting condensate is then combined with the solution passed through the heater to restore evaporated water to the process.

In evaporator 39, operating in the ranges of solution acid concentration of about 40% to 60% and solution temperature of about 220250 F. with a flue gas temperature of 230270 F. for submerged combustion, water is evaporated from the solution to attain an acid and ferrous salt concentration and temperature causing formation of a crystalline ferrous sulfate salt, preferably the monohydrate. For example, if the solution is evaporated by submerged combustion to a sulfuric acid concentration of about 50% by weight with a temperature of about 240 F, the monohydrate salt will be formed under conditions Where the solubility is approximately 10 grams per liter of solution. From the bottom of evaporator 30 concentrated pickling solution, with fine ferrous sulphate monohydrate crystals thrown down by the concentration, is continually drawn through line 55 by pump 56 for introduction at the central portion of crystallizer 31, where further crystallization and growth of ferrous sulphate monohydrate takes place. In order that the crystalline material may be removed rapidly from the environment of the burner, thereby minimizing any tendency to fuming of the acid and decomposition of the crystals in the evaporator, clear concentrated acid is returned to the evaporator from the top of the crystallizer through line 61 with control valve 62 in such volume as permits rapid circulation through the evaporator. At high operating temperature and acid concentration the anhydrous ferrous sulfate salt may be formed.

The crystallizer may be of suitable form known to the art adapted to effect crystal growth and settling of the crystals to the bottom of the apparatus for withdrawal and provided with a dam or overflow tray for discharging clarified mother liquor, that is, strong acid solution, from the top portion of the body of liquid therein. The salt settling to the bottom is drawn 01f continuously through the bottom of the crystallizer to centrifuge 32 where crystals and accompanying mother liquor are separated, the salt being discharged'to a storage bin 66 and the mother liquor being returned through line 57 by pump 58 to the crystallizer. Make-up acid may be intro duced to the crystallizer through 65, as well as at 5 9a, which may be of advantage to decrease solubility of ferrous sulfate in the crystallizer under some conditions of operation. Strong acid solution, continuously removed from the top of the crystallizer through the overflow tray, is returned at a temperature nearly that of the evaporator liquid, by pumps if necessary, through strong acid line 59, to the packed chamber 46 to effect ultimate recombination with the acid being recirculated directly through heater 8 to the pickling battery without concentration. strong acid line 59 may, however, be joined to line 7 to mix the strong acid with acid recirculated from thetanks, after the latter has passed through the primary heater 8. Hot water from tank 53 may be used to wash the centrifuged crystals and any further operations, as drying, may be used to put the monohydrate salt in desired condition. Thus, iron salts are continuously removed from the pickling solution, so that the solution passing to the battery is reconditioned both by diminishing the iron salt content and by continuously supplying heat and makeup acid. Strong acid solution may also be drawn from the crystallizer through line 64 to the strong acid storage tank 33 for storage and for delivery therefrom as required, either to all the tanks equally or in diverse amountsto individual tanks, should it be desired that different acid concentrations be used among them.

During normal pickling operations the usual flowof liquid is that indicated in the drawings by the solid arrows, The greater portion of the solution leaving the pickling battery is diverted to heater 8, the remainder to evaporator 31). Since it is desirable to avoid extensive fuming of the acid solution in the evaporator, the acid concentration is preferably kept from exceeding 50%. Therefore, control of the temperature of the solution returned to the pickling battery in a particular system involves the ratio of solution diverted to heater 8 and evaporator 30, the amount to the evaporator being increased to permit greater heat input and evaporation without exceeding the preferable 50% maximum acid concentration. The evaporator can, of course, be operated at lower acid concentration, say 45% acid with no untoward change in general operating conditions since the solubility of the monohydrate is quite low at such acid concentration. With a given heat demand, which determines the heat input at the evaporator, if it should be necessary to remove more iron salt, the solution feed to the evaporator may be decreased to drop out or precipitate more iron salts by increasing the acid concentration at which the evaporator operates.

Feeding 6066 Baurn makeup acid into the crystallizer at 65 will increase the acid concentration therein and so decrease iron salt solubility. With provision of means for removing from the flue gas the fumes evolved by sulfuric acid when heated and concentrated above the aforementioned preferable 50% concentration, the evaporator may be operated at such temperature and acid concentration that anhydrous ferrous sulfate is produced.

The continuous flow of pickling solution through the tanks is of further advantage where tubes, rods and other such shapes are pickled, since the tanks are readily designed so that the solution flow is directed lengthwise through the batches of such form in a tank to insure uniform, complete pickling of all surfaces.

if it is desirable to shut down the pickling line for any time, the valve 22 between lines 6 and 7 may be closed and valves 25 and 27 in by-pass lines 24 and 26 .opened and set as needed, to circulate solution between heater 8 and tank 21 as indicated by dashed arrows, so that the reconditioning section of the pickling system may continue to operate, with valves 14:: and 23 being closed after the tanks are drained. The pickling tanks thus, may be emptied completely while the entire .body

' storage tank '21 of sufficient capacity, the entire battery ofpickling tanks may be immediately emptied, and

.stor ed under insulated conditions, even when for some reason, the solution heating components are nottkept inoperation. 7

The specific operating conditions of the apparatus and process of course are variable as required for the pickling .solu'tion concentrations, temperature and flow required by the condition of the metal being pickled, the ranges hereinbefore given being illustrative of those useful in normal but pickling operations.

I claim: 7

1. In a'continuous process of pickling steel and the like with a heated aqueous sulfuric acid pickling solution, the cyclic steps of passing a stream of reconditioned picklingsolution produced in a subsequent step at desired temperature and acid content continuously in contact with the metal being pickled, thereby forming a used pickling solution; continuously concentrating a first portion of said used solution by direct contact flame heating to effect crystallization of a ferrous sulfate salt therein; separating the resultant concentrated solution and ferrous sulfate crystals; combining a second portion of said used solution and said concentrated solution with makeup acid and water being introduced to the process; and continuously bringing'the combined solution into intimate contact with the water vapor and combustion products arising from the concentration step to condense and absorb water vapor therefrom thereby producing and V heating said stream of reconditioned pickling solution.

2. In a continuous process of pickling steel and the like with a heated aqueous sulfuric acid pickling solution, the steps of passing a stream of reconditioned pickling solution, produced in a subsequent step at desired temperature and acid content, continuously in contact with the metal being pickled, thereby forming a used pickling solution; continuously concentrating a first portion of said used solution by direct contact flame heating to efiect crystallization of ferrous sulfate monohydrate therein; separating the resultant concentrated solution and ferrous sulfate monohydrate crystals; combining a second portion of said used solution and said concentrated solution with makeup acid and water being introduced to the process; and continuously bringing the combined solution into intimate contact with the water vapor and combustion products arising from the concentration step to condense and absorb water vapor therefrom thereby producing and heating said stream of reconditioned pickling solution.

,3. In a continuous process of pickling steel and the like with a heated aqueous sulfuric acid pickling solution, the steps of passing a stream of reconditioned pickling solution of desired temperature and acid content continuously in contact With the metal being pickled, thereby forming a used pickling solution; continuously concentrating a first portion of said used solution by submerged combustion heating to effect crystallization of ferrous sulfate monohydrate therein; continuously separating the resultant concentrated solution and ferrous sulfate monohydrate crystals; combining a second portion of said used solution and said concentrated solution with makeup acid and water being introduced to the process; continuously bringing the combined solution into intimate contact with a stream of hot water vapor and combustion products arising from the concentration step to condense and absorb Water vapor therefrom thereby producing and heating said stream of reconditioned pickling solution and producing a residual stream of partially cooled combustion products and Water vapor; and continuously bringing said residual stream into direct intimate heat exchange contact with a relatively cool water stream to heat said water stream as a source of said 'makeup water.

4. A process for regenerating a used aqueous sulfuric acid pickling solution for re-use in pickling steel and the like at desired temperature and acid concentration, comprising the steps of continuously concentrating a first portion of said used solution by submerged combustion heating to effect crystallization of a ferrous sulfate salt therein; separating the resultant concentrated solution and ferrous sulfate salt crystals; continuously combining a second portion of said used solution and said concentrated solution with makeup acid and water being introduced to the process to reestablish the desired solution concentration; and continuously bringing the combined solution into intimate contact with the water vapor and combustion products arising from the concentration step to condense and'absorb water vapor therefrom and theretill by produce a regenerated pickling solution of the temperature and acid concentration desired for re-use.

5. In the process of pickling steel and the like continuously with a heated sulfuric acid pickling solution, the steps of passinga stream of pickling solution, continuously regenerated by subsequent steps at desired temperature and acid content, in contact with metal being pickled thereby forming a used pickling solution; dividing the used solution into a first and second stream and continuously heating the first stream of the used solution for evaporating water therefrom to efiect crystallization of a ferr'oussulfate salt and to produce an evaporator gaseous effluent containing water vapor; separating ferrous sulfate salt crystals from the evaporated solution to a stream of pickling solution, continuouslyregenerated by subsequent steps at desired temperature and acid content, in contact with metal being pickled thereby forming a used pickling solution; dividing the used solution into a first and second stream and continuously heating the first stream of the used solution for evaporating Water therefrom to effect crystallization of a ferrous sulfate salt and to produce an evaporator gaseous efduent containing water vapor; separating ferrous sulfate salt crystals from the evaporated solution to produce a strong acidsolution stream; continuously mingling the strong acid solution with said second stream to form a combined stream of solution; continuously transferring heat energy and water vapor from said evaporator gaseous effluent to one of the said second and combined streams; and continuously returning the combined stream to the first step.

7. In the process of pickling steel and the like with a heated sulfuric acid pickling solution, the steps of passing a stream of pickling solution, continuously'regenerated by subsequent steps at desired temperature and acid content, in contact with metal being pickled thereby forming a used. pickling solution; dividing theused solution into a first and second stream and continuously heating the first stream of the used solution for evaporating water therefrom to effect crystallization of ferrous sulfate monohydrate and to produce an evaporator gaseous eflluent containing water vapor; separating ferrous sulfate monohydrate crystals from the evaporated solution to produce continuously a strong acid solution stream; continuously mingling the strong acid solution with said second stream to form a combined stream of solution; adding make-up acid to one of said streams; continuously transferring heat energy from said evaporator gaseous effluent to one of the said second and combined streams; and returning the combined stream to the first step.

8. In the process of pickling steel and the like with a heated sulfuric acid pickling solution, the steps of passing a stream of pickling solution, continuously regenerated by subsequent steps at desired temperature and acid content, in contact with metal being pickled thereby forming a used pickling solution; dividing the used solution into a first and second stream and continuously heating the first stream of the used solution to a temperature of from about 220 F. to about 250 F. for evaporating water therefrom to effect crystallization of a ferrous sulfate salt and to produce an evaporator gaseous efiluent containing water vapor; continuously separating ferrous sulfate salt crystals from the evaporated solution to produce a strong acid solution stream; mingling the strong acid solution with said second stream to form a combined stream of solution; continuously transferring heat energy and water vapor from said evaporator gaseous cffiuent to one of the said second and combined streams; and continuously returning the combined stream as regenerated solution to the first step.

9. In the process of pickling steel and the like with a heated sulfuric acid pickling solution, the steps of passing a stream of pickling solution, continuously regenerated by subsequent steps at desired temperature and acid content, in contact with metal being pickled thereby forming a used pickling solution; dividing the used solution into a first and second stream and continuously heating the first stream of the used solution for evaporating water therefrom to eifect crystallization of a ferrous sulfate salt from an evaporated solution of from about 40% to 60% by weight sulfuric acid concentration and to produce an evaporator gaseous efiluent containing water vapor; continuously separating ferrous sulfate salt crystals from the evaporated solution to produce a strong acid solution stream; mingling the strong acid solution with said second stream to form a combined stream of solution; continuously transferring heat energy and water vapor from said evaporator gaseous efiluent to one of the said second and combined streams; and continuously returning the combined stream as regenerated solution to the first step.

10. In the process of pickling steel and the like With a heated sulfuric acid pickling solution, the steps of passing a stream of pickling solution, continuously regenerated by subsequent steps at desired temperature and acid content, in contact with metal being pickled thereby forming a used pickling solution; dividing the used solution into a first and second stream and continuously heating the first stream of the used solution to a temperature of from about 220 F. to about 250 F. for evaporating water therefrom to efiect crystallization of a ferrous sulfate salt from an evaporated solution of from about to by weight sulfuric acid concentration and to produce an evaporator gaseous efliuent containing Water vapor; continuously separating ferrous sulfate salt crystals from the evaporated solution to produce a strong acid solution stream; mingling the strong acid solution with said second stream to form a combined stream of solution; continuously transferring heat energy and water vapor from said evaporator gaseous effluent to one of the said second and combined streams; and continuously returning the combined stream as regenerated solution to the first step.

References Cited in the file of this patent UNITED STATES PATENTS 1,392,780 Marsh Oct. 4, 1921 1,589,610 Marsh June 22, 1926 1,657,072 Dougherty Jan. 24, 1928 1,666,015 Land Apr. 10, 1928 2,668,130 Martin Feb. 2, 1954 

1. IN A CONTINUOUS PROCESS OF PICKLING STEEL AND THE LIKE WITH A HEATED AQUEOUS SULFURIC ACID PICKLING SOLUTION, THE CYCLIC STEPS OF PASSING A STREAM OF RECONDITIONED PICKLING SOLUTION PRODUCED IN A SUBSEQUENT STEP AT DESIRED TEMPERATURE AND ACID CONTENT CONTINUOUSLY IN CONTACT WITH THE METAL BEING PICKLED, THEREBY FORMING A USED PICKLING SOLUTION; CONTINUOUSLY CONCENTRATING A FIRST PORTION OF SAID USED SOLUTION BY DIRECT CONTACT FLAME HEATING TO EFFECT CRYSTALLIZATION OF A FERROUS SULFATE SALT THEREIN; SEPARATING THE RESULTANT CONCENTRATED SOLUTION 